The coercion optimiser is stressed by optimisation.
I don't think this will slow down the test runs much

Tests this patch:

  Fri Dec 11 12:01:22 GMT 2009  simonpj@microsoft.com
    * Fix two related bugs in u_tys
    
    When we normalise a type family application we must recursively call
    uTys, *not* 'go', because the latter loop is only there to look
    through type synonyms.  This bug made the type checker generate
    ill-typed coercions, which were rejected by Core Lint.
    
    A related bug only affects the size of coercions.  If faced with
      (m a) ~ (F b c)
    where F has arity 1, we want to decompose to 
       m ~ F Int,  a ~ c
    rather than deferring.  The application decomposition was being
    tried last, so we were missing this opportunity.